This invention relates to a novel process for recovering antimony trioxide from antimony sulfide ores and concentrates.
The main object of this invention is to reduce the air, water, thermal and chemical pollution inherent in present processes of making antimony trioxide.
No heat or pressure is used in the disclosed process itself and any reduced pressure or heat is used only in recycling of the alcohol and sodium or potassium hydroxide reagents.
The process is remarkably efficient in recovering antimony trioxide from antimony sulfide concentrates.
By excluding atmospheric oxygen and by the use of closed and preferably full containers and filters and with the removal of excess sulfur through decomposition of the antimony pentasulfide at temperatures above 75.degree. C, it is possible to maintain antimony in a tri-valent state of oxidation wherein no thio-antimonates are formed. Antimony, in this tri-valent state, forms little thio-antimonites, which tend to decompose to antimony tri-sulfide.
Use of the solubility differential between antimony trioxide and antimony tri-sulfide in various alcohol solutions, coupled with the addition of sodium or potassium hydroxide to the alcohol solution, coupled with the inability of antimony to form a hydroxide produces an unpredictable reaction wherein antimony trioxide is the end product.
Antimony trioxide finds utility in the manufacture of tartar emetic; as a paint pigment; in enamels and glasses; as a mordant; and in flame proofing.
The reaction of the process according to the present invention is as follows:
Sb.sub.2 S.sub.3 + 6 KOH or 6 NaOH in an ethanol, methanol or other alcohol solution, (wherein the concentration is approximately 16.66% solution of KOH, or 8% solution of NaOH) = 3 K.sub.2 S or 3 Na.sub.2 S + Sb.sub.2 O.sub.3.3H.sub.2 O. The sodium sulfide (mono) is only slightly soluble in ethanol or mehanol whereas the potassium sulfide is much more soluble.
In addition to the water of hydration formed during the initial reaction, water is found as an impurity in the antimony sulfide concentrate in the sodium or potassium reagents and in the alcohol reagents, 15.92% of the weight of Sb.sub.2 S.sub.3 in the concentrate will be water. The upper limit of the water content is 26.52%. In the presence of this quantity of water, the following reactions occur:
2 H.sub.3 SbO.sub.3 (antimonous acid -- a representation of trihydrated antimony trioxide) + 2 NaOH or 2 KOH + 2 H.sub.2 O = Na.sub.2 (or K.sub.2) H.sub.2 Sb.sub.2 O.sub.5.5H.sub.2 O. This is the sodium or potassium dihydro pyro-antimonite. Various other derivations of a compound of the same empirical formula exist through the sodium or potassium hydroxyantimonite and sodium or potassium meta-antimonite. EQU 2( (Na) or (K) Sb (OH).sub.4.H.sub.2 O) = Na.sub.2 (or K.sub.2) H.sub.2 Sb.sub.2 O.sub.5.5H.sub.2 O EQU 2( (na) or (K) SbO.sub.2.3H.sub.2 O) = Na.sub.2 (or K.sub.2) H.sub.2 Sb.sub.2 O.sub.5.5H.sub.2 O
after mixing the antimony sulfide concentrate and the alcohol solution of sodium or potassium hydroxide and with an agitation supplied from 1 to 3 minutes at temperatures not exceeding 65.degree. F the mixture is filtered. It is best to use full closed containers for the mixing and the filtration should be with limited access to atmospheric oxygen. This is to prevent any oxidation of antimony from the tri-valent state to the pentavalent state of oxidation. Filtration should be accompanied by agitation of the material to prevent caking on the sides and thus trapping some of the white precipitate material in the solid residues.
Some of the finely divided white precipitate of antimony trioxide or antimony-potassium - oxygen hydrated compounds will pass a No. 4, 1 or 2 filter paper. The bulk of the formation of these white precipitates occurs in the filtrate after it has passed the filter paper. This is due to the fact that sodium or potassium meta-antimonite is unstable in water and the remainder is due to the pairing of sodium or potassium hydroxy antimonite molecules when sufficient water is available for their hydration to permit the formation of the compound empirically written as sodium or potassium di-hydro pyro-antimonite. This compound is quite insoluble as the sodium compound and more soluble as the potassium compound, and seems to be the principal precipitate formed in the filtrate. The amount of water necessary to form this compound is an additional 66.66% of the total amount of water formed in the initial reaction.
The total time for precipitation from the filtrate is up to four hours. This time can be reduced by the addition of sufficient water to permit the required degree of hydration to occur. The ratios are:
______________________________________ Sb.sub.2 S.sub.3 + 6 KOH or 6 NaOH = 3 (K.sub.2 S) or 3 Na.sub.2 S + Sb.sub.2 O.sub.3 .multidot.3H.sub.2 O 339336240330234345 followed by: Sb.sub.2 O.sub.3 .multidot.3H.sub.2 O expressed as 2 (H.sub.3 SbO.sub.3) + 2 H.sub.2 O + 2 NaOH or 3453453680 2 KOH = Na.sub.2 (or K.sub.2) H.sub.2 SbO.sub.5 .multidot.5H.sub.2 O. 112(493 for K)(461 for Na) ______________________________________
The total amount of water required as 90 parts of water with the varius ratios to the other ingredients of the mixture (54 of the 90 is produced in the initial reaction).
There is also the hydrolysis of the potassium sulfides and sodium sulfides produced in the reaction: EQU K.sub.2 S + H.sub.2 O = KHS + KOH and Na.sub.2 S + H.sub.2 O = NaHS + NaOH
hydrolysis of the sodium or potassium sulfides occurs because the ratio of 6 KOH or 6NaOH is sufficient with 1 mole of Sb.sub.2 S.sub.3 to complete the reaction all the way to the sodium or potassium di-hydro pyro-antimonite--if there were no hydrolysis the number of sodium and potassium hydroxides in the equation would be 8. With sodium, the insolubility of the mono-sulfide in the alcohols requires a greater than 6 ratio, preferably 8. Potassium did not require a greater ratio than 6.
When formed 3 - 5 minutes have elapsed, the filtrate with the white precipitate forming in it can be centrifuged and the solids removed. The clear filtrate is then added to the residues in the filter paper and additional extractions can be made which will continue to precipitate in the filtrate after passing the filter paper. An extraction of 97% of the antimony can be made by recycling the proper amount of filtrate a fifth time. As little exposure to atmospheric oxygen as possible should be allowed in both the mixing the filtering and the re-addition of the clarified filtrate to the solid residues.
When the re-use of the clarified filtrate is ended, the solids are removed from the filtrate--as much as 4 hours may be required to completely precipitate the white substances. If the procedures are properly carried out there are no yellow formations on the filter paper from formation of thio-stibnates nor are there any red-yellow-brown residues from the decomposition of sodium or potassium thio-stibnite into antimony tri-sulfide. When the white precipitate is removed, either reduced pressure or application of heat can be used to distill the alcohol from the water solution of sodium or potassium hydroxide (the excess), the sodium or potassium sulfides and their hydrolyzed products formed in the reaction, and the arsenic sulfides and oxides (both of which are soluble in ethanol-methanol). The methanol can be distilled at 15.degree. C under 73mm of Hg pressure. The reduced pressure and applied heat should be such that the alcohol will distill but leave the water behind.
With the removal of the alcohol, the arsenic can be precipitated with a water soluble calcium salt as an insoluble calcium thioarsenite, or calcium arsenite.
The remaining water solution will contain mainly sodium or potassium hydroxide and sodium or potassium hydro-sulfide or polysulfides. The potassium compound is readily decomposed into hydrogen sulfide gas at or near the boiling point of water. The sodium equivalent requires steam at 140.degree. C to reach the same rate of decomposition.
The produced hydrogen sulfide can be burned to produce some or all of the energy required in the process. The burning of hydrogen sulfide in a limited supply of oxygen results in the formation of water and elemental sulfur.
The hydrogen sulfide can be decomposed into elemental hydrogen and sulfur at temperatures beginning at 310.degree. C. This hydrogen can then be used to replace antimony from antimony compounds which are water soluble thereby recovering elemental antimony.
The white precipitate is dried and dehydrated thermally. The temperatures employed are up to 235.degree. C. When the water has been driven off sodium or potassium hydroxide remains with antimony tri-oxide. Addition of the re-cycled and distilled ethanol or methanol to the dehydrated compound will allow the sodium or potassium to re-enter solution for use in treating additional antimony sulfide concentrate and leave the insoluble antimony tri-oxide as the product.
Antimony pentasulfide is not a naturally occurring substance. If it is present in the concentrate, heating the concentrate to 75.degree. C or higher, will reduce this pentasulfide to the tri-sulfide and elemental sulfur. It is necessary to remove this sulfur by gravity or other means (such as carbon disulfide) as either this excess free sulfur or the pentasulfide can react with the sodium or potassium sulfides produced in the reaction and form undesirable sodium or potassium thio-antimonates.
Before addition of water to the process, the total water content of the concentrate, the sodium or potassium reagents and of the alcohol should be considered. Generally, there is sufficient water available from these sources for the process. The amount of water should not exceed 10.61% of the weight of the Sb.sub.2 S.sub.3 of the concentrate.
The following examples illustrate but do not limit the invention. All the parts given are by weight unless volumes are specified.